To meet the demand for wireless data traffic, which has increased since deployment of 4th-generation (4G) communication systems, efforts have been made to develop an improved 5th-generation (5G) or pre-5G communication system. Therefore, the 5G or pre-5G communication system is also called a ‘beyond 4G network’ or a ‘post long-term evolution (LTE) system’.
It is considered that the 5G communication system will be implemented in millimeter wave (mmWave) bands, e.g., 60 GHz bands, so as to accomplish higher data rates. To reduce propagation loss of radio waves and increase a transmission distance, a beam forming technique, a massive multiple-input multiple-output (MIMO) technique, a full dimensional MIMO (FD-MIMO) technique, an array antenna technique, an analog beam forming technique, and a large scale antenna technique are discussed in 5G communication systems.
In addition, in 5G communication systems, development for system network improvement is under way based on advanced small cells, cloud radio access networks (RANs), ultra-dense networks, a device-to-device (D2D) communication, a wireless backhaul, a moving network, a cooperative communication, coordinated multi-points (COMP), reception-end interference cancellation, and the like.
In the 5G system, a hybrid frequency shift keying (FSK) and quadrature amplitude modulation (QAM) modulation (FQAM) and a sliding window superposition coding (SWSC) as an advanced coding modulation (ACM) scheme, and a filter bank multi carrier (FBMC) scheme, a non-orthogonal multiple access (NOMA) scheme, and a sparse code multiple access (SCMA) scheme as an advanced access technology have been developed.
MIMO, one of key techniques for cellular mobile communication systems, can linearly increase the spectrum efficiency of a wireless communication system by effectively exploiting the spatial-domain resources. In order to enhance the spectrum efficiency, a transmitting end may first obtain channel state information (CSI) to perform common MIMO signal processing procedures such as precoding, beamforming, etc. to use the spatial domain resources effectively. Therefore, the key to performance improvement of a MIMO system lies in obtaining accurate CSI by the transmitting end.
In a time division duplexing (TDD) system, there is channel reciprocity between uplink channels and downlink channels. Thus, uplink channel information obtained from uplink channel estimation by a base station may be regarded as the equivalent of downlink channel information. Terminals may transmit sounding reference signals (SRS) in uplink channels to assist uplink channel estimation. The SRS is generated using specific pseudo-random sequences, e.g., Zadoff-Chu (ZC) sequences. Information of the sequences is already known by the terminals and base stations. After a ZC sequence is transmitted in an uplink channel, a base station may apply coherent detection and demodulation to a received signal sequence using a corresponding ZC sequence, and obtain estimated CSI of the uplink channel. The base station may perform MIMO signal processing, such as precoding, beamforming, etc., using the obtained CSI to improve system spectral efficiency.
Design of SRS resource mapping and allocation scheme is one of most important subjects in communication systems. Long term evolution (LTE) systems based on evolved universal terrestrial radio access (E-UTRA) standard of 3rd generation partnership project (3GPP) have dedicated SRS resource mapping and allocation scheme. Since there are limited numbers of orthogonal pseudo-random sequences, pseudo-random sequences allocated to different terminals may not be strictly orthogonal to each other. Base stations may allocate SRS resources to terminals using plural multiplexing methods. Specifically, SRS resources may refer to a pseudo random sequence used by the SRS, frequency domain resources, time domain resources, or code domain resources used for transmitting the SRS, or the like. For example, multiplexing methods in the frequency domain may include comb-type pilot arrangement, frequency-hopping, etc.; multiplexing methods in the code domain may include multiple pseudo-random sequences of different cyclic shifts (CS) of the same pseudo-random sequence, or the like.
FIG. 1a is a schematic diagram illustrating a comb-type pilot arrangement. In a time slot, resources of odd-numbered sub-carriers are allocated to terminal 1 for transmitting SRS1, and resources of even-numbered sub-carriers are allocated to terminal 2 for transmitting SRS2. SRS1 and SRS2 may be the same pseudo random sequence. Although SRS resources can be reused, LTE systems are still short of SRS resources. For example, in each time slot, at most 16 full-bandwidth SRS transmissions can be performed, which uses 2 comb-type pilots and 8 different pseudo-random sequences generated using different CSs. SRS is transmitted periodically in an LTE system. In order to further increase the system capacity for SRS and to accommodate more terminals, the LTE Advanced system introduces non-periodic SRS transmission into the LTE system, which enables a base station to configure a terminal to perform only one SRS transmission instead of multiple SRS transmissions according to practical requirements.
In the same cell, a base station may allocate orthogonal SRS resources to different terminals. When the terminals transmit SRS in respective uplink channels, SRS received from different terminals by the base station are orthogonal to each other, and the base station can obtain correct channel estimation results to obtain correct CSI of each uplink channel based on the received SRS. But SRS resources allocated to terminals in different cells may be not orthogonal to each other, i.e., the SRS resources may collide with each other. For example, an LTE system may use different CSs of different ZC root sequences as pseudo-random sequences of SRS resources allocated by different cells on the same time/frequency resources. Although different CSs of the same ZC root sequence are orthogonal to each other, pseudo-random sequences obtained from different ZC root sequences are not orthogonal to each other. Root sequences are allocated to different cells according to cell IDs. Thus, SRS resources of different cells are not orthogonal to each other.
When non-orthogonal SRS resources are allocated to terminals in different cells, a base station may receive uplink SRS signals from terminals of other cells when receiving uplink SRS signals from terminals in a local cell. The SRS signals received by the base station include interference of SRS signals transmitted by terminals in other cells. This is referred to as pilot contamination.
FIG. 2 is a schematic diagram illustrating pilot contamination. When SRS1 and SRS2 are not orthogonal to each other, channel estimation performances may become worse due to the interference, and the accuracy of subsequent signal processing based on the channel estimation is also greatly reduced which reduces system capacity and spectral efficiency. When the number of antennas of a base station increases, the situation becomes worse.
Large-scale MIMO (or massive MIMO) is one of candidate techniques of the fifth generation (5G) cellular communication systems. Massive MIMO systems have sufficient spatial degrees of freedom in signal processing, thus can eliminate inter-terminal interference and inter-cell interference with low computational complexity (because only algorithms with linear computational complexity are involved). Theoretically, the uplink/downlink achievable signal-to-noise ratio (SNR) of massive MIMO systems increases with the number of antennas, thus the system capacity is increased remarkably. In practice, however, system capacity performance of massive MIMO systems is severely degraded by pilot contamination. Due to pilot contamination, massive MIMO systems may obtain poor channel estimations. The poor channel estimations may then results in severe co-channel interferences in subsequent processing, such as downlink pre-processing and uplink post-processing. The co-channel interferences may reduce or even eliminate the gain obtained from the massive antenna deployment, and the system capacity becomes interference-limited. Terminals located at the cell edge areas (simply referred to as cell edge terminals) are exposed to more serious pilot contamination than terminals located at the central area of a cell (simply referred to as cell center terminals), especially when terminals in different cells are all cell edge terminals of cells adjacent to each other. Thus, it is desirable to design a new scheme for SRS resources allocation which considers information such as different user locations, channel quality and user transmitting power to address the pilot contamination problem in massive MIMO systems and to increase system capacity.
Conventional LTE systems include TDD LTE systems and FDD LTE systems according to the different duplexing modes adopted by the systems. The TDD mode features asymmetric uplink/downlink traffic, according to which uplink/downlink time domain resources can be allocated flexibly by adjusting the uplink to downlink subframe ratio according to uplink/downlink traffic requirements. The TDD module, however, generates larger time delay, especially in processing hybrid automatic re-transmission requests (HARQ). Further, the TDD mode introduces interference between uplink and downlink, which increases the complexity of interference management. The FDD mode generates smaller time delay, and the interference management is less complex. Under the FDD mode, there is no uplink/downlink reciprocity. A base station cannot obtain CSI of a downlink channel from channel estimation of an uplink channel, and may rely on terminals to feed back CSI via additional resources, which in turn reduces system spectrum efficiency. In view of the foregoing, a hybrid division duplexing (HDD) mode is more flexible and efficient.
FIG. 3a is a schematic diagram illustrating a frame structure in an HDD system. The HDD mode integrates the TDD mode and the FDD mode. In a cell adopting pairs of carriers, a user terminal may communicate with a base station over a primary carrier and a secondary carrier according to a pre-determined communication mode. Specifically, if all of subframes in the secondary carrier are uplink subframes, the user terminal may communicate with the base station over the primary carrier and the secondary carrier under the FDD mode. If subframes in the secondary carrier are multiplexed in the time domain into uplink subframes and downlink subframes, the user terminal may communicate with the base station using downlink resources in the primary carrier and uplink resources in the secondary carrier, or downlink resources in the secondary carrier and uplink resources in the secondary carrier, under the FDD mode. An HDD system adds an uplink subframe into a downlink frame structure in conventional LTE systems for the FDD mode. The uplink subframe is for purposes such as transmission synchronization, SRS transmission, and so on. The frame structure for the TDD mode remains unmodified. The HDD mode inherits advantages of both the TDD mode and the FDD mode in LTE systems, i.e., simple CSI feedback scheme which is implemented by channel estimation, short time delay, and asymmetric uplink/downlink traffic adaptation. Thus, the HDD mode is an important duplex scheme in future 5G cellular communication standards.
In massive MIMO systems, the increased number of antennas may result in a rapid increase in the number of accurate CSI required by base stations, and there is an urgent need of obtaining CSI in massive MIMO systems. Under the HDD mode, a base station may obtain CSI from channel estimation based on previous FDD downlink frame structure in a primary carrier and TDD downlink frame structure in a secondary carrier. Further, special subframes in a frame structure in a primary carrier may be specially designed to increase SRS capacity. Thus, it is desirable for a base station in a massive MIMO system to adopt the HDD mode with a proper SRS resources allocation scheme to effectively reduce the impact of pilot contamination on system performances, achieve the gain provided by the massive antenna array, and increase system capacity.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.